Since a group III nitride semiconductor light-emitting device has a direct transition-type energy band gap which corresponds in range from the visible wavelength to the ultraviolet wavelength, and has excellent light-emitting efficiency, it has been used as a semiconductor light-emitting device, such as LEDs or LDs.
In addition, an electronic device having a group III nitride semiconductor has superior properties to those of conventional electronic devices having a group III-V compound semiconductor.
Such a group III-V compound semiconductor is generally produced by a metalorganic chemical vapor deposition (MOCVD) method using trimethyl gallium, trimethyl aluminum, and ammonia as a raw material. The MOCVD method is a method in which a carrier gas containing vapor of a raw material is supplied to the surface of a substrate, and the raw material is decomposed on the surface of the substrate heated to grow crystal of the raw material.
In the past, wafers made of a single crystal of Group III nitride semiconductor have not been marketed. In general, the Group III nitride semiconductor is obtained by growing a group III-V compound semiconductor crystal on a single crystal wafer containing a different compound from the semiconductor crystal. Therefore, there is a large lattice mismatch between the single crystal wafer containing a different compound from the semiconductor crystal and the group III-V compound semiconductor crystal obtained by epitaxial growth. For example, when gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, there is a 16% lattice mismatch between them. When gallium nitride is grown on a SiC substrate, there is a 6% lattice mismatch between them.
In general, when there is a large lattice mismatch, it is difficult to epitaxially grow crystal on a substrate directly. Even when crystal is epitaxially grown on the substrate, the density of the crystal is decreased, together with a decrease of crystallinity.
Then, when the Group III nitride semiconductor crystal is epitaxially grown on the sapphire substrate or a SiC single crystal substrate by the MOCVD method, in general, a layer, which is called a low-temperature buffer layer, and formed of aluminum nitride (AlN) or aluminum gallium nitride (AlGaN), is laminated, and then the group III nitride semiconductor crystal is epitaxially grown on the low temperature buffer layer (For example, Patent Documents Nos. 1 and 2).
In addition, a method in which a buffer layer is formed on the substrate by the sputtering method in advance, the substrate provided with the buffer layer is introduced into the MOCVD reaction furnace, and the group III nitride semiconductor layer is formed on the buffer layer, is also suggested (Patent Document No. 3). However, since the density and crystallinity of the crystal formed on the buffer layer are decreased, it is impossible to stably laminate an excellent crystal layer.
However, when the present inventors formed the buffer layer made of the above-mentioned material on the substrate by the sputtering method, and the gallium nitride-based compound semiconductor was laminated on the substrate provided with the buffer layer according to the Patent Documents Nos. 1 and 2, there was a limitation for improving the crystallinity of the gallium nitride-based compound semiconductor.
The reasons may be because the buffer layer contains amorphous phases or polycrystal phases in Patent Documents Nos. 1 and 2.
In the lamination methods using aluminum nitride, which is laminated by the sputtering as the buffer layer, disclosed in Patent Documents Nos. 3 and 3, due to the difference in lattice mismatch between the buffer layer and the gallium nitride layer, it is not possible to improve the crystallinity.    [Patent Document No. 1] Japanese Patent (Granted) Publication No. 3026087    [Patent Document No. 2] Japanese Unexamined Patent Application, First Publication No. H4-297023    [Patent Document No. 3] Japanese Patent (Granted) Publication No. 3440873    [Patent Document No. 4] Japanese Patent (Granted) Publication No. 3700492